1. Field of Invention
The present invention is directed to methods of applying an additive to the surface of a toner created by the emulsion/aggregation (EA) technique. In particular, the reduction of relative humidity (RH) sensitivity of the toner is accomplished by applying the additive to the surface of the toner during a fluidized bed spraying procedure.
2. Description of Related Art
EA polyester toner particles are very hydrophilic, and thus susceptible to poor or unpredictable triboelectric charging upon exposure to atmospheric humidity. More in particular, EA polyester toners have hydrophilic functional groups on the surface of the toner, causing humidity sensitivity. Such toner particles thus need to be treated with a hydrophobic agent in order to perform over a wide range in humidities.
Low humidity is frequently referred to as C-zone (approximately 20% relative humidity), and high humidity is frequently referred to as A-zone (approximately 80% relative humidity). In practical use, this is referring to the humidity of the environment during use of a printer. This difference leads to large RH sensitivity ratios, which means that the toner is effective in low humidity conditions but not in high humidity conditions. For reference purposes, the RH sensitivity ratio of untreated EA polyester toner particles can range from approximately 5 to 12. The ultimate goal is for the RH sensitivity ratio to be as close to one as possible. When such an RH sensitivity ratio is achieved, the toner is equally effective in both high humidity and low humidity conditions. Said another way, the toner has low sensitivity to changes in RH.
One method of improving RH sensitivity of EA polyester toner has included forming a hydrophobic shell around the toner particles. However, this method has not proven viable because fusing characteristics of the toner are too adversely affected.
Another method of improving RH sensitivity is to treat EA polyester toner particles with a hydrophobic agent in the wet chemistry stage to improve RH sensitivity as described in U.S. Pat. No. 6,143,457 to Carlini et al., which is incorporated herein by reference in its entirety. The wet chemistry method is a surface treatment method that occurs after toner particles have been grown to the desired size by the EA technique. During the wet chemistry method, surface additives are added to EA polyester toners prior to any removal of the liquid from toner mixture, i.e., additives are added to the toner slurry.
The wet chemistry method is very limiting. In practice, as the wet chemistry treatment proceeds, the more hydrophobic treated toner will precipitate from the solution before the treatment is complete, and will agglomerate. This presents a problem because once the toner particles agglomerate, it is very difficult to separate them, particularly if the temperature is high enough to cause particles to fuse together.
The EA polyester toner particles are hydrophilic because of sulfonate groups on their surface. As part of the wet chemistry method, a phosphonium bromide salt, such as stearyltributylphosphonium bromide (STBP), is added to mask the sulfonate groups. During this wet chemistry method, an ion exchange occurs, and the resulting toner has hydrophobic stearyl phosphonium groups deposited/bonded on its surface.
Once the polyester toners are coated with a hydrophobic surface additive, the water is removed from the toner slurry. The toner particles are usually approximately 10 to 15 percent of the total weight of the toner slurry. The first step in removing the water is a filtration step. The product collected in the filter is called a wet cake which consists of the toner and some residual water. The water in the wet cake is approximately 40 percent of the total weight of the wet cake. The wet cake is the product of any filtration method, not just the wet chemistry method.
Once the wet cake has been produced, it is still necessary to remove the remaining water to be able to effectively use the created toner particles. This is usually done by a fluidized bed or equivalent drying equipment.
Another problem of the wet-chemistry method is that the additive has to be soluble in water. This means that significant quantities of water are necessary to dissolve the additive. This requires additional reactor capacity, which is expensive. Furthermore, a relatively high temperature is necessary to keep the additive dissolved in solution. However, a high temperature may cause the toner particles to start to coalesce and deviate from the desired particle size and particle size distribution.
The problems of the wet chemistry method have resulted in a continued processing problems of the treated EA polyester toners. In particular, the wet chemistry additive application process of the prior art has resisted scale-up to date and is cumbersome and time consuming.